reporter-bugzilla: if adding to existing BZ, clearly flag comment as a dup.

Partially fixes 864891.

With this commit, the added BZ comment will look like this:

Another user experienced a similar problem:

<comment field goes here>

reporter:       libreport-2.1.9.9.gdef1.dirty
backtrace_rating: 4
cmdline:        will_segfault
crash_function: main
executable:     /usr/bin/will_segfault
kernel:         3.10.11-100.fc18.x86_64
package:        will-crash-0.4-1.fc18
reason:         will_segfault killed by SIGSEGV
runlevel:       N 5
type:           CCpp
uid:            0

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>